Compare materials under the conditions your part will actually face. Expanded PTFE often offers high air permeability and water repellency, while polyurethane films may suit different bonding or cost needs. Neither material wins every application. Check the supplier’s test method, not just its headline rating. Airflow should be measured at a stated pressure difference; water-entry pressure should be reported with the test fluid and sample setup. A membrane that performs well on a flat coupon may behave differently after welding into a small, curved housing.

Design details matter. Look at active membrane area, adhesive width, vent placement, and protection from dust, oil, and direct spray. IEC 60529 defines IPX7 enclosure testing as temporary immersion in water up to 1 metre for 30 minutes. That rating applies to the tested enclosure, not automatically to a loose membrane. For road vehicles, ISO 20653 provides a vehicle-specific framework for dust and water protection. Ratings are useful, but only when the assembled component is tested under relevant conditions. That comparison is not always tidy.